Pandemics (B) Flashcards
Malaria
Malaria is a big obstacle to further global progress
- Still close to #1 infectious cause of child mortality
- Access to available interventions remains poor
- Drug and insecticide resistance rife
- Poor predictors of severe malaria
- No vaccine and desperate for new drugs
- malaria is considered one of the key drivers of poverty (countries cant get rich unless they get rid of malaria first)
Taxinomic classification of Malaria
- Phylum: Apicomplexa
- Genus: Plasmodium
- 5 species infect Humans: P.falciparum (main one), and P. vivax
- Many other species infect animals
- single celled protozoan parasite, all apixomplexa invade cells like viruses
Malaria is more common just ___ and ___ of the Equator
North and south
- where it becomes more and more tropical, malaria more common because their life cycle extends all year round, winters stop them breeding
How important is Malaria now?
- 1 bil people worldwide with parasites in their blood but only 2-300mil clinical cases
(they’re not sick but the biomass of these parasites is a heavy burden) - 0.5mil deaths/year (mostly children)
- Malaria seriously hinders social and economic development
>est $12 bil in direct costs
>10% of yearly household spending in Africa
>work and school absenteeism (as high as 28% in some areas)
The definitive host of the malaria parasite is the:
Anopheles mosquito
RBCs stain ____ colour when they are infected with malaria parasites
Blue
Life cycle of P. falciparum
Three stages:
- Mosquito stage (sexual reproduction)
- Liver stage (asexual reproduction)
- Blood stage (asexual reproduction, major amplification stage)
only in blood stage does it cause disease
In the mosquito gut, there are ___ and ___ versions of the parasite
Male and female
-diploid organ, fertilisation
- in mosquito salivary glands, you get miosis into haploid organisms called sporozoites (which enter the human host)
Gamete>Zygote>Ookinete>Sporozoite
Sporozoites enter the:
Human liver and infects hepatocytes (100 - 1000 sporozoites injected)
Doesnt cause disease for these 1-2 weeks in the liver
Merozoites enter the:
RBCs where they start to replicate and keep amplifying
important: go from relative small numbers up to 1% of all of your blood cells
- started off with injection of 10-100 sporozoites but end up with 1% of all RBC about 10^12 cells infected
Important aspects of lifecycle and spread
- Sexual stage in the female anopheles mosquito (1-2 weeks)
- Injected sporozoites enter hepatocytes via skin (~30 mins)
- Asexual liver stage (1-2 weeks)
- Asexual blood stage is relatively synchronous and takes 2-3 days
- Disease occurs a week to a month after infection
- Gametocytes form in the blood and are taken up by a feeding mosquito
Malaria genome has 5200 genomes encoding for a lot of proteins, but the one we are interested in is:
PfEMP1
(Plasmodium falciparum Erythrocyte Membrane Protein 1)
Confers virulence and immune escape
Disease only occurs in the ______
Blood-stage
- Fever, chills, anaemia
(hard to predict which individuals move on to develop more serious complications) - Most deaths (95%) caused by P.falciparum
>Cerebral malaria, coma
>Severe anaemia
>Placental malaria - P.vivax also results in significant morbidity but low mortality
>relapsing malaria (hypnozoites in liver)
P. vivax has a _____ form
Latent
- can live as a form called a hypnozoite in the liver for life
- for reasons not understood, these hypnozoites can reactivate later in life and cause disease
How does the malaria parasite avoid splenic clearance?
Cytoadherence/sequestration
- Infected RBC is stuck to BV wall, taken itself out of circulation
- Parasites are maturing inside, and across the course of 48h, parasites grown within there and burst out
What is splenic clearance?
Spleen recognises RBCs that are not normal anymore
- has cross-hatched fenestrations like a little filter, RBCs go through spleen and are squeezing through these fenestrations
- Thought that the spleen is testing those cells (looking for cells that are getting too old and wants to get rid of them e.g. if a cell is getting too rigid)
Clearance mechanism
- Macrophage-like system will be activated and clear those red cells
- A plasmodium infected RBC wants to stay away from the spleen»_space; cytoadherence
Cytoadherence is sometimes associated with ______ pathologies
Severe pathologies
- depending on which BVs they sequester to (not all BVs are the same, not all RBCs are the same)
e. g. Cerebral malaria - infected brain capillary
What microscopic feature is diagnostic of malaria parasites?
BV full of cells with a gold particle in them - digested heme (haemozoan)
The parasite inside the RBC is digesting the haemoglobin, eating the protein partly because it is a nutrient source and partly because it needs to create space inside this RBC to grow
Heme - iron calated structure, very toxic for all cells including parasites
Malaria parasite polymerises that digested heme into this gold crystal called haemozoan
Why are there different cytoadherence patterns and different clinical outcomes of Plasmodium infection?
Parasites can cytoadhere to BVs of different tissues
- BV in brain not same as BV in liver or fat, or kidneys, or skeletal muscle
- Those BV walls are all different and some p.falci parasites like BVs in one organ and others will like them in another organ
Cytoadherence is caused by a single molecule:
PfEMP1
- responsible for binding a receptor on the endothelial cell wall
- can bind different receptors
- Parasite encode different PfEMP1s, 60 different copies of the gene encoding PfEMP1, and depending on which of those genes is expressed, will determine which receptor the parasite will bind to on the microvascular endothelium of BVs
What does PfEMP1 bind to?
Receptors on the microvascular endothelium
- e.g. CD36, some tissues rich in this receptor, other receptors expressed on other tissues
- ICAM1 is thought to be enriched in the brain so if you had a parasite expressing PfEMP1 that liked ICAM1, it would sequester in the brain BVs (can cause cerebral malaria) and CD36 causes sequestering in the muscle BVs
Knob structure?
PfEMP1 is exported all the way to the RBC cell surface for cytoadherence
What causes antigenic distinct waves of parasitaemia seen in a single person infected with a single parasite?
With P.falci, after a week or so, you get another wave oi parasitaemia, that then gets cleared, then another wave, then clearance - why?
- each of these parasites, even though 100% genetically identical, are switching around the gene encoding for PfEMP1, using a different one each time
- What you get is immunity against the parasite expressing the PfEMP1 that causes antigenic wave A, then it switches to express B, then you develop immunity against B, then it switches to C and so on
Different antigenic distinct waves of parasitaemia = different disease symptoms, why?
A and B might cause mild disease where the sequestering is to fat BV, but C maybe sequesters to brain BVs or another important organ then you get really severe disease
Other thing you can see is that this mechanism allows parasites to persist a long time
- P. falci doesnt have a latent form but it wants to hang around in the human host as long as possible so it can get taken up by a mosquito (then goes on to infect another person)
___ genes encode PfEMPs
Var genes
~60 copies per genome
- only one is expressed at any one time
- var genes are scattered around the 14 chromosomes, particularly clustered at the ends
Immune response against sequestered RBCs involve:
Antibodies directed against the PfEMP1 which stops them binding to BV walls (generated within a week or two)
In malaria endemic areas with lots of P. falciparum, children get infected with malaria a lot, then stop getting sick from the age of 5 onwards. They still get infected but wont get ill. Why?
Grow up, women start to have babies, all of a sudden become susceptible to malaria again
- get really sick, both themselves and their unborn baby
- parasites bound to walls of placental lining
- parasites packed full in the placenta where there is a problem
- turns out these parasites encode a PfEMP1 that binds to CSA and sequester to this point
Why do these women who have gotten infected hundreds of times before not gotten immunity?
- there has never been a selective force in most of these women to develop immunity against those CSA binding parasites
- Women with first pregnancy, and men always, have never had a placenta
How are most var genes ‘silenced’ while allowing just one to be expressed?
Chromosome ends cluster at the nuclear periphery and are in a ‘silent’ heterochromatic state
- if you look in the nucleus of a single parasite, and use a fluorescent probe for the ends of chromosomes, see just 3 or 4 spots
- 14 chromosomes so there should be 28 chromosome ends, but only see 3 or 4 spots
- Research showed that if you use stronger and stronger detergents, you start to see more of those spots
- Turns out that the ends of chromosomes are tightly clustered together so it looks like only have 3-4 ends
- Not only clustered, but at the outer edge periphery of the nucleus > wrapped up in heterochromatin > way to physically wrap up genes
- tight bundle of DNA
- limits accessibility of the DNA to be transcribed into RNA (hide RNA transcription binding sites)
Describe how the var2CSA gene gets activated
Silent var2CSA on Chr12 is bound up with other chromosomes in heterochromatin
If there is CSA selection, Chr12 gets separated from the heterochromatin bundle and move into an active zone to get transcribed
>active var2CSA
var gene silencing is caused by:
Epigenetic control of transcription (driven by physical histone modifications to DNA)
Histone modification at the nuclear periphery generally has genes inaccessible to transcription factors
A particular localisation is associated with a new histone modification that permits transcription
*stochastically (randomly), one of the 60 genes will move into active site for transcription, but selection allows a particular population to grow up
